Study I: Thrombotic thrombocytopenic purpura (TTP) is a rare disorder of uncertain etiology that causes multi-organ dysfunction due to occlusion of small blood vessels by microthrombi. Over the past three decades plasma therapy has reduced the mortality rate from nearly 100% to about 20%. Although most patients recover from the initial episode, the relapse rate is high, and TTP remains a disease with substantial morbidity and mortality. Recent discovery of an association between decreased yon Willebrand factorcleaving protease (vWCP) levels and disease activity together with the demonstration of an autoantibody directed at this protein in some patients, raise the possibility that additional immunomodulatory therapy may be of benefit. We propose a randomized study comparing plasma exchange to plasma exchange plus corticosteroids to test the hypothesis that corticosteroids will increase the rate of durable complete responses. We also propose to determine if splenectomy will decrease the subsequent relapse rate in patients who have experienced a relapse of TTP. Studies correlating vWCP levels to disease activity and additional laboratory studies characterizing other possible disease correlates are planned. Study II: Prevention strategies for platelet alloimmunization and accompanying platelet transfusion refractoriness are not completely effective, particularly in individuals previously exposed to HLA and platelet antigens through transfusion of non-leukocyte reduced blood products or pregnancy. Selecting platelet products to avoid a refractory patient's alloantibody response is a mainstay of transfusion support for these patients but there is no consensus on which selection strategy is most effective for this clinical problem. We propose to compare the effectiveness of platelet crossmatching with HLA matching in providing platelet transfusion support to patients who are refractory to random donor platelet transfusions. In this context we will also determine the extent to which factors unrelated to HLA or platelet-specific alloimmunization (non-immune factors, ABO incompatibility) influence platelet transfusion responses. This information will be extremely useful in designing effective and cost-efficient platelet transfusion protocols for refractory patients.